I read that concave points in a Frost diagram disproportionate. But what are the products of the disproportionation?

For example, in the Frost diagram for manganese at pH = 0, the species $\ce{HMnO4-}$ is concave with respect to $\ce{MnO4-}$ and $\ce{H3MnO4}$. But at the same time, it is also concave with respect to $\ce{MnO4-}$ and $\ce{MnO2}$. How do I know which set of products $\ce{HMnO4-}$ disproportionates to in acidic solution at pH = 0?

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1 Answer 1


Interesting question. First of all understand what is on the x-axis and on the y-axis. You y-axis is Gibbs free energy/Faraday constant. Choose three consecutive oxidation states, the middle oxidation state, if it lies above the two, is likely to disproportionate.Read more here on Frost and Latimer digram

Frost diagram

In your case look at the example of $Mn^{3+}$, if you connect $Mn^{2+}$ and $MnO_2$, $Mn^{3+}$ lies above the two, so 3+ oxidation of Mn is unstable with to disproportionation.

In short, just by using Frost diagram you cannot predict disproportionation of Mn (VII) because if it were to disproportionate, you will need Mn(VIII), which is not shown, so Frost diagram analysis only works for three points and you cannot say anything about the boundary values.

  • $\begingroup$ I am asking about the products of the disproportionation of the +7 oxidation state. $\endgroup$ Commented Feb 7, 2020 at 17:24
  • $\begingroup$ It will $not$ disproportionate, as I said, it will work with three points not an isolated boundary value. $\endgroup$
    – ACR
    Commented Feb 7, 2020 at 17:35

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